The present invention relates to drive control arrangements for motor graders and in particular, relates to an all wheel drive arrangement for motor graders and other vehicles.
All wheel drive arrangements for motor graders have the advantage that the front drive wheels can compensate for poor traction conditions experienced by the tandem drive. The front wheel drive can operate in a passive mode where the front wheels only form part of the primary drive when there is slippage of the tandem drive. This passive mode basically allows the front wheel drive to respond when slippage has occurred on the tandem drive while in good traction conditions, the grader acts as if it is only driven by the tandem drive.
All wheel drive systems allow a variation in the level of aggression of the front wheel drive and the front wheel drive can be set to be faster than the tandem drive. This aggressive mode is not the most cost efficient manner of operating the grader but it is desirable for certain applications.
Existing front wheel drive arrangements for motor graders use a hydraulic motor which is supplied with hydraulic fluid under pressure for providing the necessary drive of the front wheels. There is a common hydraulic pump that supplies hydraulic fluid to each of the hydraulic motors and a flow control valve distributes the amount of hydraulic fluid to each motor. Control between the two motors is based on control of the distributing valve. Various sensors are used for sensing the speed of the front wheels, rear wheels, ground speed and other parameters which sensors are connected to a controller which controls the speed of the pump as well as the position of the distribution valve. With this arrangement, fine adjustment between the drive of the front wheels is difficult to achieve.
The present invention in a preferred embodiment uses a different drive arrangement where each front wheel has its own hydraulic circuit and is hydraulically separated from the hydraulic circuit of the other drive. This drive arrangement provides increased control and sensitivity in adjustment of the system.
In a further embodiment a bypass valve is used to control the switching of the hydraulic circuit across the motor from an open circuit to a closed circuit. This by-pass valve automatically adjusts to changing conditions and appropriately switches the conditions of the drive arrangement.
The present invention allows an all wheel drive which additional works in a creep mode where only the front wheels are driven.
The particular drive arrangement for each front wheel drive allows simplified control logic due to the automatic response of the by-pass valve to changing hydraulic conditions as opposed to electrically driving such a by-pass valve to respond to changing conditions. The particular hydraulic circuit automatically responds to the changing conditions and causes the hydraulic circuit to appropriately respond.